In recent years, wearable devices have been developed progressively with the spread of Internet of Things (IoT). Representative examples thereof include a watch and glasses that can be connected with internet. Wearable devices that can always monitor physical conditions are also necessary in a medical field and a sports field, and are expected to be a growth field in the future.
Wearable devices include a form that is adhered to a body to monitor physical conditions constantly. Such a wearable device is generally composed of a bio-electrode to detect electric signals from a body, wiring to send the electric signals to a sensor, a semiconductor chip to be the sensor, and a battery. Normally, an adhesive pad is necessary to be adhered to skin. The structures of a bio-electrode, wiring around the same, and an adhesive pad are specifically described in Patent Document 1, In the wearable device described in Patent Document 1, a silicone based adhesive film is disposed around the bio-electrode, and the bio-electrode is connected with a sensor device by stretchable silver wiring in the shape of bellows coated with a stretchable urethane film.
Urethane films have high stretchability and strength to possess excellent mechanical properties as coating films for stretchable wiring. The urethane film, however, has hydrolytic properties to cause a disadvantage of lowering the stretchability and the strength due to hydrolysis. On the other hand, silicone films are free from hydrolytic properties, but has a disadvantage of lower strength.
Accordingly, it has been conducted to investigate silicone-urethane polymers with each molecule having both of a urethane bond and a siloxane bond. The cured product of this polymer has higher strength than single silicone, and lower hydrolytic properties than single polyurethane. The cured product of this polymer, however, fails to equal the strength of single polyurethane and the repellency of single silicone, only giving strength and repellency in the middle of those of silicone and polyurethane.
On the other hand, a material in which polyurethane and silicone are blended has been investigated. For example, Patent Document 2 and Patent Document 3 describe a material in which non-reactive silicone and crosslinkable polyurethane are blended. In a film formed from such a material, silicone comes up to the surface of a cured polyurethane film, (bleed out) to improve the repellency of the film surface. In such a film, however, the silicone is not crosslinked, which causes peeling of silicone on the film surface to be tend to lower the repellency.
Patent Document 4 describes a material that contains crosslinkable urethane-acrylate and silicone-acrylate are blended. This material can improve the heat resistance by blending silicone-acrylate, and can form a cured product with higher strength and adhesiveness by blending urethane-acrylate. In this material, however, a solvent is not blended. Patent Document 5 describes a material that contains crosslinkable urethane-acrylate, silicone-urethane-acrylate, and a solvent having low boiling point such as ethanol and methanol. In this material, however, the ratio of the silicone-urethane-acrylate is high relative to the ratio of the urethane-acrylate. Patent Document 6 also describes a material that contains crosslinkable urethane-acrylate and silicone-urethane-acrylate are blended. In this material, however, a solvent is not blended.
As described above, the materials in which crosslinkable urethane-acrylate and silicone-(urethane)-acrylate are blended have been investigated previously. In the cured products of these materials, however, the urethane-acrylate and the silicone-(urethane)-acrylate are cured with the both being dispersed uniformly with each other. Accordingly, they can only give intermediate strength and repellency between those of silicone and polyurethane.
Patent Document 7 describes a material in which crosslinkable urethane-acrylate, silicone-urethane-acrylate, and a crosslinkable solvent are blended. In curing of material like this, however, the crosslinkable solvent, containing a polymerizable double bond, crosslinks simultaneously with crosslinking of the urethane-acrylate and the silicone-urethane-acrylate, which cause curing with the urethane-acrylate and the silicone-urethane-acrylate being dispersed uniformly with each other. Accordingly, the cured product only gives intermediate strength and repellency between those of silicone and polyurethane. Moreover, 2-ethylhexylacrylate, which is used as the crosslinkable solvent, has a boiling point above 200° C. to cause slower evaporation rate.